The proteasome has been validated as a therapeutic target, as demonstrated by the FDA approval of bortezomib, a boronic acid proteasome inhibitor, for the treatment of various cancer indications, including multiple myeloma. However, other more highly proteasome-specific inhibitors that could have fewer toxic side effects have recently been described. These compounds include peptide epoxy ketones such as epoxomicin, described in U.S. Pat. No. 6,831,099, the contents of which are hereby incorporated by reference, and those described in U.S. Pat. No. 7,232,818, the contents of which are hereby incorporated by reference. However, the low aqueous solubility of some of these compounds makes it difficult to formulate compositions at sufficiently high concentration to enable practical administration with desired antineoplastic or other pharmacological effects. Thus, additional methods of formulating peptide epoxy ketones are needed.